1. Field of the Invention
The present invention relates to a method for preparing elongated-shaped silica sol. More specifically, the silica sol prepared by the method of the present invention is characterized by a particular shape of colloidal silica particle and displays an excellent filming or coating property after being coated and dried on a solid article and therefore it is utilized as a coating composition and in other various fields.
The method according to the present invention prepares the silica sol at high efficiency.
2. Description of Related Prior Art
A known process for producing elongated-shaped silica sol particles is disclosed in JP-A-1-317115 (corresponding to U.S. Pat. No. 5,221,497). The method provides a stable aqueous alkaline silica sol having SiO.sub.2 concentration of 1 to 6% by weight, and it comprises mixing an aqueous solution containing a water-soluble calcium salt or magnesium salt or a mixture of the calcium salt and the magnesium salt with an aqueous colloidal liquid of active silicic acid containing from 1 to 6% by weight of SiO.sub.2 concentration, and further mixing an alkali metal hydroxide or a water-soluble organic base or a water-soluble silicate of said alkali metal hydroxide or said water-soluble organic base with the mixed solution at a molar ratio of SiO.sub.2 /M.sub.2 O of from 20 to 300 (where SiO.sub.2 represents the total amount of the silica content derived from said active silicic acid and the silica content of said silicate and M represents an alkali metal atom or a molecule of said organic base) and heating the resulting mixture in the temperature range of from 60.degree. to 300.degree. C. for a duration of from 0.5 to 40 hours.
In the process for producing elongated-shaped silica sol particles as disclosed in JP-A-1-317115, however, the colloidal silica particles are bonded and grown by heating them under a condition that water is hardly evaporated. Thus, the resulting silica sol contains SiO.sub.2 in a concentration range of from 1 to 6% by weight. The silica sol obtained as a result is generally concentrated thereafter to a SiO.sub.2 concentration range of from 5 to 30% by weight. In the concentration-step, a higher concentration of SiO.sub.2 before the concentration-step allows more efficient production of highly concentrated silica sol as a final product. However, where the SiO.sub.2 concentration of silica sol is in the range of from 1 to 6% by weight, the efficiency drops because much time and energy are consumed in the concentration-step.